Various information recording techniques have been developed following the increase in volume of information processing in recent years. Particularly, the areal recording density of a HDD (hard disk drive) using the magnetic recording technique has been increasing at an annual rate of about 100%. Recently, the information recording capacity exceeding 160 GB has been required per 2.5-inch magnetic disk adapted for use in a HDD or the like. In order to satisfy such a requirement, it is necessary to realize an information recording density exceeding 250 Gbit/inch2.
In order to achieve the high recording density in a magnetic disk for use in a HDD or the like, a magnetic disk of the perpendicular magnetic recording system (perpendicular magnetic recording disk) has been proposed in recent years. As a perpendicular magnetic recording medium for use in the perpendicular magnetic recording system, there has been proposed a perpendicular magnetic recording medium or a CGC medium using CoCrPt—SiO2 as a magnetic recording layer because it exhibits high thermal stability and excellent recording characteristics.
As a first background art, in order to improve the recording density in such a perpendicular magnetic recording medium, it is necessary to reduce the noise in a magnetization transition region of the magnetic recording layer (improve the S/N ratio). For that, it is necessary to improve grain separation and miniaturization of magnetic crystal grains of the magnetic recording layer.
As a method for improving the grain separation and miniaturization of the magnetic crystal grains of the magnetic recording layer, there is, for example, a method such as adjusting the composition of a target for use in a process of forming the magnetic recording layer by a sputtering method or increasing the pressure of a gas (film forming gas pressure) for use in the film formation. However, in terms of the recording density required in recent years, it is difficult to sufficiently miniaturize the magnetic crystal grains only by adjusting the composition of the target. Further, when the film forming gas pressure is increased, there is, for example, a possibility that the structure of the magnetic recording layer is affected so that sufficient reliability as a magnetic recording medium cannot be ensured.
Herein, use is conventionally made of a structure in which an underlayer adapted to control the orientation of magnetic crystal grains of a magnetic recording layer is formed under the magnetic recording layer. There is conventionally known a structure in which a layer of Ru—SiO2 or the like is used as the underlayer (see, e.g. Patent Documents 1 and 2). In this structure, the miniaturization of the magnetic crystal grains of the magnetic recording layer is facilitated by miniaturizing crystal grains of the underlayer.
As a second background art, the easy magnetization axis of a magnetic recording layer is oriented in a plane direction of the surface of a substrate in the conventional in-plane magnetic recording system (also called the longitudinal magnetic recording system or the horizontal magnetic recording system), while the easy magnetization axis is adjusted to be oriented in a direction perpendicular to the surface of a substrate in the perpendicular magnetic recording system. As compared with the in-plane magnetic recording system, the perpendicular magnetic recording system can more suppress a thermal fluctuation phenomenon during high-density recording and thus is suitable for increasing the recording density.
Conventionally, CoCrPt—SiO2 or CoCrPt—TiO2 is widely used as a magnetic recording layer, wherein Co forms crystals with a hcp structure (hexagonal closest packed crystal lattice) and Cr and SiO2 (or TiO2) are segregated to form grain boundaries. By using such a material, since SiO2 (or TiO2) is segregated around ferromagnetic Co, physically independent fine Co grains tend to be formed so that high recording density tends to be achieved.
In general, an underlayer is provided for improving the crystal orientation of the magnetic recording layer. Ti, V, Zr, Hf, or the like is known as the underlayer, but as shown in Patent Document 3, Ru (ruthenium) is currently predominant. This is because it is known that Ru takes a hcp structure and effectively improves the perpendicular orientation of the easy magnetization axis of the magnetic recording layer composed mainly of Co (cobalt) to enhance the coercive force Hc, thereby achieving an increase in recording density with a predetermined S/N ratio and a predetermined resolution ensured.
It is known that even if the material is the same, the underlayer changes in film function depending on an atmospheric gas pressure in a film forming process. Patent Document 4 proposes a structure having, as an undercoat film of a perpendicular magnetic layer, a layer containing ruthenium and formed in a high-pressure argon atmosphere and a layer containing ruthenium and formed in a low-pressure argon atmosphere. In Patent Document 4, it is described that the layer containing ruthenium and formed in the low-pressure argon atmosphere (around 1 Pa) exhibits an effect for higher orientation of the magnetic layer, while the layer containing ruthenium and formed in the high-pressure argon atmosphere (about 6 Pa to 10 Pa) exhibits an effect for finer grains of the magnetic layer.
As described above, in order to achieve the increase in recording density, it is effective to miniaturize the magnetic grains. However, if the magnetic grains are excessively miniaturized, the number of atoms forming each magnetic grain becomes too small, so that the thermal fluctuation phenomenon arises as a problem like in the in-plane magnetic recording medium. In order to avoid this thermal fluctuation problem, the following method has been employed so far.
Specifically, it is a method of further providing a pre-underlayer under an underlayer and optimizing a material or a film structure of the pre-underlayer to facilitate the alignment of the orientation of crystal grains of the underlayer, thereby indirectly improving the crystal orientation of a magnetic recording layer to improve the coercive force. The material of the pre-underlayer can be selected from various materials such as, for example, Ni (nickel), Pt (platinum), and Pd (palladium).
As a third background art, in order to achieve high recording density in a magnetic disk for use in a HDD or the like, it is necessary to miniaturize magnetic crystal grains forming a magnetic recording layer serving to record an information signal, and further, to reduce the thickness of the layer. However, in the case of a conventionally commercialized magnetic disk of the in-plane magnetic recording system, as a result of the advance in miniaturization of magnetic crystal grains, there has arisen the so-called thermal fluctuation phenomenon where the thermal stability of a recorded signal is degraded due to superparamagnetism so that the recorded signal is lost, which has thus become an impeding factor for the increase in recording density of the magnetic disk. In order to solve this impeding factor, a magnetic disk of the perpendicular magnetic recording system has been proposed.
In the case of the perpendicular magnetic recording system, as is different from the in-plane magnetic recording system, the easy magnetization axis of a main recording layer is adjusted to be oriented in a direction perpendicular to the surface of a substrate. As compared with the in-plane magnetic recording system, the perpendicular magnetic recording system can suppress the thermal fluctuation phenomenon. Further, a soft magnetic layer is provided in a perpendicular magnetic recording medium to enable convergence of the magnetic flux from a recording head by the soft magnetic layer and it is possible to generate a sharp and large magnetic field as compared with an in-plane magnetic recording medium by the mirror image effect, and therefore, the perpendicular magnetic recording system is suitable for increasing the recording density.
When the magnetic recording layer has a hcp structure (hexagonal closest packed structure) in the perpendicular magnetic recording system, the easy magnetization axis is a c-axis so that it is necessary to orient the c-axis in the normal direction of the substrate. In order to improve the orientation of the c-axis, it is effective to provide a nonmagnetic underlayer with a hcp structure under the magnetic recording layer. A CoCr alloy, Ti, V, Zr, Hf, or the like is known as a material forming the underlayer, but it is known that particularly Ru (ruthenium) can effectively improve the crystal orientation of the magnetic recording layer to enhance the coercive force Hc.
In the perpendicular magnetic recording system, the S/N ratio (Signal/Noise Ratio) and the coercive force Hc are improved by forming the magnetic recording layer into a granular structure in which a nonmagnetic substance or an oxide (see, e.g. Patent Document 5) is segregated to form grain boundary portions between magnetic grains of the magnetic recording layer, thereby isolating and miniaturizing the magnetic grains. Further, in order to facilitate the miniaturization of the magnetic grains, it is important to miniaturize grains of the underlayer formed under the magnetic layer, and thus, a nonmagnetic substance or an oxide is added also to the underlayer (see, e.g. Patent Document 6).
Patent Document 1: JP-A-2002-334424
Patent Document 2: JP-A-2006-85742
Patent Document 3: JP-A-H7-334832
Patent Document 4: JP-A-2002-197630
Patent Document 5: JP-A-2003-036525
Patent Document 6: JP-A-2006-085742